Global baby making seems at no want for a polish, with the world population at 7 billion and rising. But in reality, some evidence suggests part of this picture is breaking down: Sperm may be changing for the worse — at least in some places. The decline has been blamed on everything from cellphones in pockets to hormones in water to fatty food in the Western diet.

Evidence for a drop in sperm quality and quantity has included anecdotal reports from sperm banks as well as larger scientific studies. For instance, one sperm bank in Israel says that when it opened its doors 1991, it turned away about a third of the applicants for low quality. Using the same standard today, it would reject more than 80 percent, according to an article in the LA Times. And while the jury is still out on whether there is a real "sperm decline" and what that means for fertility, scientists say if the little swimmers are truly changing, it may be a red flag for harmful environmental toxins or even physiological changes in the human body.

"I firmly believe there's a decline," said Grace Centola, a sperm bank consultant and president-elect of the Society for Male Reproduction and Urology. Centola said she combed through the past eight years of sperm-donor data in the Boston area, and found "a statistically significant decline in semen volume, sperm count and motility over those years." The age of sperm donors didn't change, so the drop couldn't be attributed to age — and even the technicians who took the samples remained the same. Centola says she will present the data at a conference this fall. [5 Myths About Fertility Treatments]

(Other U.S. sperm banks contacted by LiveScience said they hadn't noticed any changes in sperm quantity or quality.)

Counting sperm

The story globally is far from clear. Twenty years ago, a paper published in the British Medical Journal reviewed 61 studies of semen quality carried out between 1938 and 1990 and came to a jolting conclusion: In 50 years, the sperm counts had halved — going from 113 million sperm per milliliter to 66 million sperm per milliliter. (The World Health Organization considers 15 million sperm per milliliter to be a normal sperm concentration.)

But there was a problem, as the studies reviewed only looked at developed countries and may have included people who were already concerned about their sperm count and were turning up for fertility studies to begin with.

To make matters worse, the baselines for sperm count aren't consistent, and there is nearly no data available before about 1950. A Danish study showed no decrease in the count or quality of sperm in 5,000 men enrolled in military service, while a recent study of men in Finland showed that men born toward the end of the 1980s tended to have lower sperm counts than those born at the beginning of the same decade. In Israel, sperm banks report that sperm quality has plummeted over the past 10 to 15 years — the concentration of sperm in samples collected by the bank dropped 37 percent, according to the LA Times.

"Semen quality certainly appears to be declining in the regions and in the populations that are traditionally studied," said Raywat Deonandan, an assistant professor and epidemiologist at the University of Ottawa, referring to western, developed countries.

Deonandan's research, detailed online March 22 in the International Journal of General Medicine, suggests many previous semen-quality studies suffer from selection bias; they tended to take samples from more affluent men in more urban areas.

The upshot of sperm decline

So why care about the muddy picture, if babies are still being born? So far, there has been no global shortage of babies — but in 30 percent of the cases of infertility, there is a male factor, said Wendie Robbins, a professor at the UCLA School of Nursing. Male infertility is suspected in about 70 percent of cases in Israel.

"Many times, there is just no cause that people can find for infertility," she said, adding that she was surprised how interested the men in a new study of hers were about increasing their fertility. "People underestimate how much men are interested in optimizing the possibilities for their offspring." (Robbins and colleagues recently found in a study partially funded by the California Walnut Commission that eating walnuts may boost sperm quality.)

Deonandan says there are two reasons why the sperm situation should be taken seriously. "If the decline is real, then an essential aspect of the human animal is being changed very rapidly in only a few generations," he told LiveScience.

The cause of such a decline could be a canary in a coalmine for other human health problems. If the mechanism is hormonal, linked, for example to an increase of estrogen from plants like soy, then it means that other aspects of human health are also being affected; hormonal systems regulate much of physical and psychological health. If the cause is environmental — pesticides, diet, or even cellphones — then industries could make changes to prevent damaging sperm. Centola gives another possible cause, linked more to behavior than the environment: sexually transmitted infections can hinder sperm production and motility. [Quiz: Test Your STD Smarts]

Studying sperm in men from remote places, like the Pacific islands, said Deonandan, would give a clearer picture of the cause of this drop. Those men are less likely to be exposed to industrial pollutants, less likely to eat so-called modern or Western diets, high in processed fats and simple carbohydrates, and more likely to be involved in hard physical labor.

"In other words, they are less likely to engage in what most of us would consider the modern, Western lifestyle," he said. If those men don't show a decline in sperm quality, then the reason for the drop could be behavioral or environmental, but not a fundamental, genetic change in human physiology.

"Rapid changes in reproductive function may indicate serious changes in our environment, which may be affecting our health in so far undetectable ways," said Deonandan. So, while the global decline in sperm quality may not stand up to rigorous testing, "it's definitely worth taking seriously, since it may open the door to deeper insights into other ways in which changes to our environment, behavior and lifestyles are negatively affecting our biologies."